Leukocytes with ring‐shaped nuclei (ring cells) are present in bone marrow (BM; ~50% of BM cells), in peripheral blood (PB), and in inflammatory infiltrates of mice, but also in humans during myeloproliferative disorders. They are usually referred to as polymorphonuclear cells (PMN), but we demonstrate that they additionally encompass different types of mononuclear (MNC)‐like ring cells. PMN ring cells had constricted ring‐shaped nuclei with a wide cytoplasmic center and were sorted among the GR‐1^high fraction. The MNC‐like ring cells belonged to the GR‐1^low fraction. Their nuclei were not segmented and the cytoplasmic center of their nuclei was small. They were heterogeneous with one subgroup containing monocytes/macrophages according to ultrastructure, immunophenotype (BM8, F4/80, CD13, ER‐HR3), activity of unspecific esterase, and phagocytosis of Leishmania major. A second subgroup contained myeloic precursor cells as they proliferated (Ki67), expressed ER‐MP12, and showed on ultrastructure distribution patterns of peroxidase activity compatible with myelocytes, promyelocytes, or promonocytes. A third subgroup of cells had large, sometimes lobulated nuclei, was lineage marker^negative/low (GR‐1, Mac‐1, B220 etc.), CD38⁻, but c‐kit⁺ and sca‐1⁺, and thus belonged to a close progeny of murine hematopoietic stem cells. In PB, ring cells encompassed mainly PMN, but also monocytes and cells with characteristics of both the granulocytic and monocytic lineage. Thus, ring cells comprise mature and precursor forms of myeloic cells. Their analysis revealed that in mice a clear distinction between the granulocytic and monocytic lineage beyond the GM‐CFU stage is not always feasible. J. Leukoc. Biol. 65: 217–231; 1999.